Ventricular fibrillation of the heart is characterized by fine, rapid, fibrillatory movements of the ventricular muscle that replace the normal cardiac contraction. Since very little pumping action occurs during ventricular fibrillation, the situation is fatal unless quickly corrected by cardiac conversion. During conversion, defibrillation level electrical energy is applied to the heart in an attempt to depolarize the myocardial tissue of the heart and allow a normal sinus rhythm to be reestablished.
One theory that has been proposed to explain the mechanism of conversion by the application of defibrillation electrical current is the critical mass hypothesis. The critical mass hypothesis suggests that it is not necessary to halt all fibrillation activity in order to have defibrillation occur, but that it is sufficient to halt only a "critical mass" (perhaps 75%) of the myocardium in the ventricles. In this theory, the assumption is made that if all fibrillation activity is localized to a region smaller than the critical mass of myocardium, the remaining fibrillation activity is not capable of maintaining fibrillation and will die out after one or two cycles, resulting in normal sinus rhythm.
Implantable cardioverter/defibrillators (ICDs) have been successfully used to treat patients who have experienced one or more documented episodes of hemodynamically significant ventricular tachycardia or ventricular fibrillation. The basic ICD consists of a primary battery, electronic circuitry to control both the sensing of the patient's cardiac signals and the delivery of electrical shocks to the patient's heart, and a high-voltage capacitor bank housed within a hermetically sealed titanium case. One or more catheter leads having defibrillation electrodes are implanted within the heart of the patient or on the epicardial surface of the patient's heart. The catheter leads are then coupled to the implantable housing and the electronic circuitry of the ICD and are used to deliver defibrillation level electrical energy to the heart.
It has been suggested that a minimum and even (i.e., similar in all parts of the ventricles) potential gradient generated by a defibrillation level shock is necessary for effective cardiac defibrillation. This potential gradient is affected, and thus determined, by the voltage of the shock and the electrode configuration employed. It has also been suggested that a maximum potential gradient also exists that, beyond this value, deleterious electrophysiological and mechanical effects may occur, such as new arrhythmias, myocardial necrosis, or contractile dysfunction. Therefore, how and where defibrillation electrodes are placed on and/or within the heart has a major effect on whether or not a critical mass of cardiac tissue is captured during a defibrillation attempt.
Endocardial defibrillation catheters, those not requiring a thoracotomy to be place on the heart, have a major advantage over the epicardial lead systems by reducing the morbidity, mortality, and cost of thoracotomy procedures. However, a major problem with these systems is the potential for high defibrillation thresholds as compared to system employing epicardial defibrillation electrodes. Changes to the waveform of the defibrillation shock and to the combinations of endocardial leads implanted into a patient and the current pathways used can result in efficacious defibrillation therapy being delivered to the patient.
The easiest and most convenient way to perform the implantation of a fully transvenous system is to use only one endocardial lead with both sensing and pacing and defibrillation capabilities. One such endocardial lead is sold under the trademark ENDOTAK C (Cardiac Pacemaker, Inc./ Guidant Corporation, St. Paul, Minn.), which is a tripolar, tined, endocardial lead featuring a porous tip electrode (placed in the apex of the right ventricular) that serves as the cathode for intracardiac right ventricular electrogram rate sensing and pacing, and two defibrillation coil electrodes, with the distal one serving as the anode for rate sensing and as the cathode for morphology sensing and defibrillation which the proximal coil electrode positioned within the superior vena cava functions as the anode for defibrillation.
However, single body endocardial leads used for both defibrillation and rate sensing have been reported to suffer technical inadequacies that may pose significant risks to the patient. Endocardial electrograms obtained from integrated sense/pace-defibrillation leads have been shown to be affected after shock delivery, with their amplitude decreasing to such a significant degree that arrhythmia redetection is dangerously compromised. As already mentioned above, obtaining adequate defibrillation thresholds has been a major problem with the nonthoracotomy endocardial lead systems. Therefore, a need exists to design an endocardial lead system that effectively reduces defibrillation thresholds and allow for reliable post-defibrillation shock sensing and pacing.